鉛-銀-銻-碲 四元熱電材料系統之相平衡、微結構與熱電性質

Phase equilibria, microstructures and thermoelectric properties of the quaternary Pb-Ag-Sb-Te thermoelectric material system

吳欣潔

指導教授 ： 陳信文

國立清華大學/工學院/化學工程學系/博士(2012年)

摘要

在永續發展與環境保護的前提下，熱電材料被視為重要的能源材料之一。四元鉛-銀-銻-碲(Pb-Ag-Sb-Te)為高效能熱電材料系統，其中包括四元合金AgPbmSbTe2+m、三元化合物AgSbTe2及二元化合物PbTe等，都曾被報導具極佳的熱電轉換效率。為進一步開發新穎熱電材料與開拓熱電元件應用，本研究致力於建構此四元系統及其子系統(銀-銻-碲、銀-鉛-碲與鉛-銻-碲)之相圖、探討其微結構之變化與熱電特性之關係。以實驗方法建構銀-鉛-碲(Ag-Pb-Te) 三元系統之液相線投影圖，其中的三元共晶點(組成為Ag-4.3at%Pb-62.6at%Te，反應為: L=Ag5Te3+PbTe+Te)形成具方向性層狀微結構(lamellae)，200-600nm的PbTe相均勻生成於此層狀基材。此銀-鉛-碲三元共晶點為頗佳之熱電材料，具極低之熱傳導係數(0.3 W/mK)，且熱電優質在400K可達0.41。本研究亦建構銀-銻-碲(Ag-Sb-Te)三元系統於400℃及250℃之等溫相圖及液相線投影圖。其中，三元相AgSbTe2穩定存在400℃等溫相圖中，且具一定的溶解度範圍(49.0-53.0at%Te 及28.0-30.0at%Sb)及非化學劑量比例之平均組成 (Ag20Sb30Te50)；並以穿透式電子顯微鏡對三元相AgSbTe2進行相分析與晶體結構之鑑定。在三元銀-銻-碲液相線投影圖中，確認了具奈米級生成相的三元共晶點(L=Ag2Te+AgSbTe2+δ-Sb2Te，496.5℃)，200-600nm的Ag2Te相均勻分布在AgSbTe2與δ-Sb2Te組成的基材中；以穿透式電子顯微鏡對三元銀-銻-碲共晶材料進行高解析影像分析，可鑑定出三個不同的相區，分別為AgSbTe2、Ag2Te及δ-Sb2Te；透過慢速的單方向凝固法(0.3mm/hr) 研究此三元共晶材料的固化行為，其相鑑定結果亦與電鏡分析結果一致。為進一步了解此共晶結構的生長機制，並探討其共晶點延伸致四元空間中的行為，本研究以實驗方法建構了四元系統鉛-銀-銻-碲的液相線投影圖(於36at%Te定組成下)。另外，將第四元元素(鉛)添加至三元相AgSbTe2中，形成四元合金(組成為PbxAg20Sb30-xTe50 x=3,4,5 及 6)，並透過單方向凝固法合成具高密度、高純相(AgSbTe2)的片狀樣品。其中，以單方向凝固法生成的5at%Pb 與6at%Pb合金具有極低的熱傳導系數(κ)，約為0.3~0.4 W/mK；檢視其微結構可發現，其存在有晶界的不均勻性與奈米級的析出物(precipitates)。5at%Pb樣品之熱電優質(zT)可達0.7-0.8 (425K)，經過熱處理後，同樣組成樣品(5at%Pb)的熱電優質下降至0.4。判斷為熱處理步驟使得晶粒的尺寸增加、組成不均勻性降低，致使材料中的中長波聲子(phonon)繞射減少、熱傳導係數上升。

關鍵字

熱電材料 ； 鉛-銀-銻-碲 ； 相平衡 ； 微結構 ； 熱電性質

並列摘要

Thermoelectric material has been recognized as a promising candidate in the application of sustainable energy. The quaternary Pb-Ag-Sb-Te system has attracted much attention because it contains high-zT AgPb18SbTe20 (zT>2 at 800K), PbTe and AgSbTe2. The phase stability, microstructural evolution and resultant electrical/thermal transport properties of quaternary Pb-Ag-Sb-Te and the constituent systems (Ag-Sb-Te, Pb-Sb-Te and Ag-Pb-Te) are investigated. The liquidus projections of both ternary Ag-Pb-Te and Pb-Sb-Te systems are constructed. In particular, the Ag-Pb-Te ternary-eutectic alloy, with composition of Ag-4.3at%Pb-62.6at%Te, forms a partially aligned nano-sized lamellar microstructure, which comprises both the PbTe and Ag5Te3 phases, and an additional dotted PbTe of 200-600nm. This particular ternary-eutectic alloy is unidirectionally solidified using the Bridgman method, resulted in a nanostructured composite with an extremely low thermal conductivity(κ) of 0.3 W/mK and a zT peak of 0.41 at 400K. The phase diagrams of ternary Ag-Sb-Te system are constructed as well, including the 400℃ and 250℃ isothermal sections and the liquidus projection. The ternary AgSbTe2 is stabilized at 400℃ but not at 250℃, with homogeneity region of 49.0-53.0at%Te and 28.0-30.0at%Sb. The nano-scaled microstrucutre and crystal structures of the non-stoimeteric AgSbTe2 are analyzed by the transmission electron microscope (TEM). In particular, an ordered array of nano-wire microstructure, comprising a 200nm Ag2Te and a matrix of AgSbTe2+δ-Sb2Te, was resulted from a Class I reaction: L=AgSbTe2+Ag2Te+δ-Sb2Te with liquid composition of Ag-40at%Sb-36.0at%Te at 496.5℃. To understand and guide production of uniform bulk samples of this composite, the liquidus projection of quaternary Pb-Ag-Sb-Te system at 36.0at%Te isoplethal section is constructed experimentally using quenched samples. High-resolution transmission electron microscopy (TEM) confirms that these three phases are simultaneously present at the nanometer scale. Furthermore, unidirectional solidification experiments of the ternary eutectic alloy using the Bridgman method are carried out to examine the alloy's solidification behaviors. Pb-alloyed AgSbTe2 (PbxAg20Sb30-xTe50, x=3,4,5 and 6) are also unidirectionally solidified using the modified Bridgman method. The as-solidified 5at%Pb and 6at%Pb alloys, which exhibit high phase purity of AgSbTe2, contain grain-boundary inhomogeneity and nano-precipitates of δ-Sb2Te, leading to an extremely low thermal conductivity (κ) of 0.3-0.4 W/mK. A peak zT of 0.7-0.8 is found in as-solidified 5at% specimen at 425K. However, after annealing at 673K, the zT peak of 5at%Pb(annealed) decreases to 0.4, presumably due to increase in grain size and decrease in inhomogeneity.

並列關鍵字

Thermoelectric materials ； Pb-Ag-Sb-Te ； Phase equilibria ； Microstructure ； Thermoelectric properties

參考文獻

1. M. S. El-Genk, H. H. Saber, and T. Caillat. ” Efficient segmented thermoelectric unicouples for space power applications.” Energy Conversion and Management , Vol. 44, pp. 1755-72, (2003).

2. C. D. Kramer.” Thermoelectric applications as related to biomedical engineering for NASA Johnson space center.” Materials Research Society Symposium Proceedings, 478 (Thermoelectric Materials-New Directions and Approaches), pp. 309-314, (1997).

3. T. Ikeda, S. M. Haile, V. A. Ravi, H. Azizgolshani, F. Gascoin, and G. J. Snyder, "Solidification processing of alloys in the pseudo-binary PbTe-Sb2Te3 system" Acta Mater., Vol. 55, pp. 1227-39, (2007).

4. F.J. DiSalvo. “Thermoelectric cooling and power generation.” Science, Vol. 285, pp.703-706, (1999).

6. B. Du, H. Li, J. Xu, X. Tang, and C. Uher, “Enhanced Figure-of-Merit in Se-Doped p-Type AgSbTe2 thermoelectric compound.” Chem. Mater., Vol. 22(19), pp. 5521-5527, (2010).